1. Field of the Invention
The present invention relates to a demagnetizing circuit including a positive temperature coefficient (PTC) thermistor device employed in association with a cathode ray tube, e.g., in a color television receiver.
2. Description of the Prior Art
Generally, in an electronic devices employing a cathode ray tube, there is provided a demagnetizing circuit for eliminating the magnetic field produced around the cathode ray tube.
A typical PTC thermistor device 5 of the above described type is shown in FIG. 1, which comprises a positive temperature coefficient (PTC) thermistor 1 formed in the shaped of plate, a pair of electrodes 1a and 1b deposited on opposite faces of the PTC thermistor 1, a pair of lead terminals 2 and 3 made of metal and having their one ends 2a and 3a so bent as to provide a spring effect, and a casing 4 made of electrically non-conductive material. The lead terminals 2 and 3 are supported by the casing 4 and are positioned as to locate their bent ends 2a and 3a inside the casing 4. The PTC thermistor 1 is provided in the casing 4 and held tightly between the bent ends 2a and 3a for effecting an electrical connection of the electrodes 1a and 1b with the terminals 2 and 3, respectively.
FIG. 2 shows a demagnetizing circuit including the thermistor device 5 described above, a demagnetizing coil 6, a power switch 7 and a demagnetizing power source 8, which are connected in series to define a closed loop. The demagnetizing coil 6 is positioned close to an element which carries a residual magnetism.
When the switch 7 is turned on, an a.c. current (referred to as a demagnetizing current) flows through the PTC thermistor device 5, resulting in an gradual increase of temperature of the PTC thermistor 1. Thus, the resistance of the PTC thermistor 1 gradually increases and, as a result, the demagnetizing current attenuates, as shown by a waveform A in FIG. 5. The demagnetizing current flowing through the coil 6 excites a gradual decreasing a.c. magnetic field, which serves to remove the residual magnetism in the element positioned close to the coil 6.
Although the PTC thermistor device 5 shown in FIG. 1, and its circuit shown in FIG. 2 are simple in construction, the demagnetizing current remains to a considerable degree after 3 seconds and even after 3 minutes from the turning-on of the switch 7 as shown in a chart 1 below.
CHART 1 ______________________________________ Initial Current Current current 3 sec. later 3 min. later ______________________________________ FIG. 2 10 A 70 mA 30 mA circuit FIG. 4 10 A 25 mA 2 mA circuit ______________________________________
It is to be noted that the results shown in the chart 1 is obtained when a.c. 100 volt is supplied from the demagnetizing power source 8.
The remaining demagnetizing current results in an incomplete demagnetization of the element to be demagnetized and, thus it adversely produces a distortion of a picture on the cathode ray tube.
In order to reduce the remaining demagnetizing current, an improved PTC thermistor device 17, as shown in FIG. 3, has been proposed. The PTC thermistor device 17 shown in FIG. 3 comprises two PTC thermistors 11 and 12, in which the PTC thermistor 11 is deposited with electrodes 11a and 11b and the PTC thermistor 12 is deposited with electrodes 12a and 12b. The PTC thermistor 11 and 12 are positioned on opposite sides of a terminal plate 13 inside a casing 16. The terminal plate 13 has a lead portion 18 extending outwards from the casing 16. The PTC thermistor 11 is tightly held between the terminal plate 13 and a bent portion 14a of a lead terminal 14, and the PTC thermistor 12 is tightly held between the terminal plate 13 and a bent portion 15a of a lead terminal 15, in a similar manner to that described above.
FIG. 4 shows a demagnetizing circuit employing the PTC thermistor device 17 described above. As shown in FIG. 4, the demagnetizing power source 8 and the switch 7 are connected in series between the lead terminals 14 and 18, and the demagnetizing coil 6 is connected between the terminal leads 14 and 15.
When the switch 7 is turned on, an a.c. current flows through the PTC thermistor 11 and, at the same time, another a.c. current, i.e., the demagnetizing current, flows through the PTC thermistor 12 resulting in an increase of the temperature of the PTC thermistor 11 and 12. The heat generated from the PTC thermistor 11 is transferred to the PTC thermistor 12 through the terminal plate 13. Thus, the resistance of the PTC thermistor 12 increases rapidly and, as a result, the demagnetizing current attenuates more rapidly than the previous circuit of FIG. 2, as indicated in the chart 1. More particularly, after 3 seconds from the time the switch is closed, the demagnetizing current attenuates to 25 mA, and after 3 minutes from the same, it attenuates to 2 mA.
Although these figures show a great improvement, they are not sufficient for a delicate device such as a cathode ray tube.
In order to further attenuate the demagnetizing current, one approach is to further increase the temperature of the PTC thermistor 12, through which the demagnetizing current flows. To meet this end, one may attempt to reduce the size of the PTC thermistor device 17 to reduce the heat dissipation area. However, such a small size demagnetizing device 17 has the disadvantage that the current peaks are dropped by a considerably great degree B, shown in FIG. 5, to cause an undesirable magnetization.